1. Field of the Invention
The present invention relates to an interferometer which is used, for example, in the highly accurate measurement of the shape of a surface to be detected.
2. Description of Related Art
The shape measurement of a surface to be detected by the use of the light interference method has been widely used as an extremely highly accurate measuring technique. In the past, interferometers of the type utilizing two light beams, e.g., those called as the Twyman-Green type and the Fizeau type have been used as the conventional apparatus for making the measurement of such kind. All of these types of interferometers are so degined that the light beam from a light source is divided into two light beams including a measuring light and a reference light and, after providing the proper optical path difference (phase difference) between the light beams, the light beams are combined to interfere with each other. When measuring the shape of a surface to be detected, one of the divided two light beams is reflected as a measuring light from the surface so that the reflected light, which is diffracted in response to the irregularities of the surface, is imaged on an observation surface through an imaging lens and this reflected light is interfered with the other light beams (a reference light) reflected from a reference surface, thereby observing the interference fringes produced in accordance with the irregularities of the surface to be detected.
Usually, in the Twyman-Green interferometer the reference surface and the surface to be detected are arranged on the different optical paths so that the two light beams or the measuring light and the reference light are separated into the different optical paths and then they are again combined, thereby causing an interference. On the other hand, in the Fizeau interferometer a surface to be detected and a semitransparent reference surface are arranged on the same optical path.
With such interferometer of the type utilizing two light beams, the resolution of the surface irregularities observed on the basis of the interference fringes is determined by the angular aperture of an imaging lens and the finer interference fringes can be observed with an increase in the angular aperture. This angular aperture is determined by the diameter of an aperture stop arranged at the Fourier transform image plane of the surface to be detected.
With such above-mentioned conventional interferometer, however, the aperture stop diameter of the imaging lens is fixed as such and the resolution of the observable irregularities of the surface to be detected is constant. Thus, where it is desirable to observe such surface irregularities with various spatial resolutions freely there is the disadvantage of being unable to meet the requirement.